Electrochemical cells

ABSTRACT

An electrochemical cell has an inner, titanium-rod electrode ( 1 ) mounted coaxially within an outer, titanium-tube electrode ( 2 ) with a porous, ceramic tube ( 3 ) mounted coaxially between them to define coaxial, annular passageways ( 4,5 ) for liquid flow in separate streams lengthwise of the cell between respective pairs of inlet/outlet ports ( 6, 6; 7, 7 ). A cup-shape fitting ( 8 ) having a stepped-down internal diameter is clamped onto the rod electrode ( 1 ) at each end of the cell, with the tubular electrode ( 2 ) at that end held tightly sealed in the mouth ( 14 ) of the fitting ( 8 ). Each end of the ceramic tube ( 3 ) projects into the larger-diameter cavity-part ( 10 ) of the fitting ( 8 ) at that end and has a radial flange ( 17 ) that provides a sliding seal within this cavity-part ( 10 ) for keeping the inlet/outlet ports ( 6,7 ) for the respective liquid streams at that end, divided off from one another as well as allowing the ceramic tube ( 3 ) limited freedom for longitudinal sliding relative to the electrodes ( 1, 2 ).

This invention relates to electrochemical cells.

The invention is particularly, though not exclusively, concerned withelectrochemical cells of the kind used for purification of water and theproduction of disinfecting aqueous solutions. Electrochemical cells ofthis kind have been proposed in which an ion-permeable, ceramic membraneis interposed between coaxial electrodes, and it is one of the object ofthe present invention to provide an improved form of such a cell.

According to the present invention there is provided an electrochemicalcell in which an inner cylindrical electrode extends coaxially within atube of ion-permeable material that is mounted coaxially within ahollow, cylindrical outer electrode to define inner and outer annularpassageways between the respective electrodes and said tube forliquid-flow-lengthwise of them from one to the other of cup-shapefittings at either end of the cell, each cup-shape fitting having twoinlet/outlet ports that communicate with the inner and outer passagewaysrespectively, wherein each cup-shape fitting defines a cylindricalcavity which has an open-mouth that is of a diameter to receive arespective end of the outer electrode for liquid-tight sealingtherewith, said tube projects at each end of the cell from within theouter electrode into said cavity of the fitting at that end and has anannular rim for engagement with an internal cylindrical wall of thecavity for liquid-tight sealing therewith to separate the inner andouter passageways from one another within the fitting, and the innerelectrode projects at each end of the cell from said tube into thecavity of the fitting at that end.

The outer electrode may be a metal tube, and the inner electrode a metalrod. The metal involved in each case may be titanium, however, where theelectrode is for use as the cathode of the cell it may, as analternative, be of stainless steel. Where the electrode is for use asthe anode, it may have a coating (for example, of ruthenium and iridiumoxides) that acts as a catalyst in the electrochemical operation of thecell.

The ion-permeable tube may be porous and in this respect may be of aceramic material, for example composed of aluminium, zirconium andyttrium oxides. The annular rim at each end of the cell may be slidablewithin the cavity of the fitting at that end to enable sliding of theion-permeable tube relative to the inner and outer electrodes, and maybe provided by a flange of a member that is sealed or otherwise securedto the ion-permeable tube at that end. The extent to which theion-permeable tube can slide relative to the inner and outer electrodesmay be limited by abutment within the cavity at each end of the cell.The abutment may be with a part (for example, a pin or annular rim) thatprojects laterally into the cavity, or of a longitudinal projection fromthe tube-end with the bottom of the cavity, or of a longitudinalprojection from the bottom of the cavity with the tube-end.

The cavity of each cup-shape fitting may have a stepped internaldiameter for defining a first cavity-part leading from the open mouthand a second cavity-part of smaller diameter opening from the firstcavity-part. In these circumstances, the ion-permeable tube may projectat each end of the cell from within the outer electrode into the firstcavity-part of the fitting at that end to have its annular rim engagewith the internal cylindrical wall of the first cavity-part. Theinlet/outlet ports of each fitting may open into/from the first andsecond cavity-parts respectively.

An electrochemical cell in accordance with the present invention willnow be described, by way of example, with reference to the accompanyingdrawing, in which:

FIG. 1 is a part sectional side-elevation of the electrochemical cellaccording to the invention; and

FIG. 2 is an enlarged sectional side-elevation of one end of theelectrochemical cell of FIG. 1.

Referring to FIGS. 1 and 2, the inner and outer electrodes of theelectrochemical cell are formed by a coaxial rod 1 and tube 2respectively. The rod 1 and tube 2 are of titanium and the insidesurface of the tube 2, which is to act as the anode, is coated withruthenium and iridium oxides that act as a catalyst in theelectrochemical operation of the cell. A porous ceramic tube 3 composedof aluminium, zirconium and yttrium oxides, is supported coaxiallybetween the rod 1 and tube 2 to provide an intermediate ion-permeablemembrane for separating the products derived electrochemically at thetwo electrodes. More particularly, the tube 3 divides the space betweenthe rod 1 and tube 2 into two coaxial, annular passageways 4 and 5 forliquid-flow lengthwise of the electrodes formed by the rod 1 and tube 2.

The passageways 4 and 5 terminate at either end of the cell insideways-extended inlet/outlet ports 6 and 7, respectively, ofindividual one-piece plastics fittings 8 (for example, of polypropylene)that are retained on opposite ends 9 of the rod 1. Each fitting 8 is ofcup-shape with a stepped internal diameter that defines a firstcylindrical cavity-part 10 into/from which the port 7 opens, and whichleads via an annular end-face 11 (FIG. 2) into a second cylindricalcavity-part 12 of smaller diameter into/from which the port 6 opens. Theends 13 of the tube 2 are lightly machined externally over a shortlength (for example, 5 mm) so as to enable them to be received andprovide liquid-tight sealing, in recessed mouths 14 (FIG. 2) of thecavity-parts 10 of the fittings 8.

The rod 1, on the other hand, extends axially through both cavity-parts10 and 12 of each fitting 8, and has ends 9 of reduced diameter. The end9 within each fitting 8 projects from the cavity-part 12 into andthrough a bore 15 of the fitting 8. It is a liquid-tight interferencefit within the bore 15 (it may be lightly machined over a length of, forexample, 5 mm, to ensure a constant diameter for the purpose), and isscrew-threaded to receive a nut 16 (only one shown, in FIG. 1) where itprojects from the fitting 8. The nuts 16 are tightened to clamp thefittings 8 firmly onto the two ends 13 of the tube 2 and hold them tothe rod 1 with the rod 1 and tube 2 coaxial with one another.

The tube 3, which projects beyond the ends 13 of the tube 2 within thecavity-part 10, has a radial rim or flange 17 (FIG. 2) at each end. Eachflange 17 is part of a plastics tubular moulding 18 which is retained onthe tube 3 by a plastics sleeve 19 that is heat-shrunk onto therespective end of the tube 3; the dimensioning of the moulding 18 allowsfor manufacturing tolerances in tube-diameter. The flange 17 provides aliquid-tight, sliding fit within the internal cylindrical wall of therespective cavity-part 10, separating the passageways 4 and 5 from oneanother within the fitting 8 at that end.

More especially, the extent of projection of the tube 3 beyond the end13 of the tube 2 within each fitting 8 ensures that the flange 17 islocated deeper within the cavity-part 10 than the port 7 so that itmaintains appropriate separation of the port 7 from the cavity-part 12and port 6. The integrity of this separation is maintained since thefreedom of the tube 3 to slide relative to the tube 2 is limited to asmall distance (for example, 1 to 2 mm) by the walls 11 at both ends ofthe cell. However, the freedom for sliding ensures that the clampingforces exerted by the nuts 16 clamping the fittings 8 to the ends 13 ofthe tube 2 and holding them on the ends 9 of the rod 1, are not exertedon the tube 3. The fact that the somewhat-fragile, ceramic tube 3 is notsubject to any clamping force avoids the need to cushion it resiliently.

The attachment of the mouldings 18 to the tube 3 may be by adhesiveinstead of the heat-shrunk sleeves 19, or by bonding them on by heatingthe ends of the tube 3 so that the plastics is partially melted as themouldings 18 are pushed on.

The form of construction of the electrochemical cell described above,has significant advantages of simplicity and economy in that the numberof components is reduced as compared with earlier forms, and the extentof machining required is light and limited to the ends 9 of the rod 1and the ends 13 of the tube 2. Moreover, stock material may be used forthe electrodes (for example seam-welded tubing may be used for the tube2), and the porous ceramic tube 3, which for example, may have aporosity between 50% and 70% with a pore size of between 0.3 and 0.5microns, is not required to withstand compressional forces or satisfytight dimensional tolerance-limits. The flanges 17 of the mouldings 18attached to the ends of the tube 3 provide both the sliding freedom andthe liquid-tight sealing required, in a simple manner without any strictdimensional requirement on tube-diameter.

1. An electrochemical cell in which an inner cylindrical electrodeextends coaxially within a tube of ion-permeable material that ismounted coaxially within a hollow, cylindrical outer electrode to defineinner and outer annular passageways between the respective electrodesand said tube for liquid-flow lengthwise of them from one to the otherof cup-shape fittings at either end of the cell, each cup-shape fittinghaving two inlet/outlet ports that communicate with the inner and outerpassageways respectively, wherein each cup-shape fitting defines acylindrical cavity which has an open-mouth that is of a diameter toreceive a respective end of the outer electrode for liquid-tight sealingtherewith, said tube projects at each end of the cell from within theouter electrode into said cavity of the fitting at that end and has anannular rim for engagement with an internal cylindrical wall of thecavity for liquid-tight sealing therewith to separate the inner andouter passageways from one another within the fitting, and the innerelectrode projects at each end of the cell from said tube into thecavity of the fitting at that end.
 2. An electrochemical cell accordingto claim 1 wherein the outer electrode is a metal tube.
 3. Anelectrochemical cell according to claim 1 or claim 2 wherein the innerelectrode is a metal rod.
 4. An electrochemical cell according to anyone of claims 1 to 3 wherein the particular one of the electrodes thatis to act as the cathode of the cell, is of titanium or stainless steel.5. An electrochemical cell according to any one of claims 1 to 4 whereinthe particular one of the electrodes that is to act as the anode of thecell is of titanium.
 6. An electrochemical cell according to claim 5wherein the anode electrode has a coating that acts as a catalyst in theelectrochemical operation of the cell.
 7. An electrochemical cellaccording to claim 6 wherein the coating of the anode electrode is ofruthenium and iridium oxides.
 8. An electrochemical cell according toany one of claims 1 to 7 wherein the ion-permeable tube is of a porousceramic material.
 9. An electrochemical cell according to claim 8wherein the ceramic material is composed of aluminium, zirconium andyttrium oxides.
 10. An electrochemical cell according to any one ofclaims 1 to 9 wherein the annular rim at each end of the cell isslidable within the cavity of the fitting at that end to enable slidingof the ion-permeable tube relative to the inner and outer electrodes.11. An electrochemical cell according to claim 10 wherein the annularrim at each end of the cell comprises a flange of a member that issealed or otherwise secured to the ion-permeable tube at that end. 12.An electrochemical cell according to claim 10 or claim 11 wherein theextent to which the ion-permeable tube can slide relative to the innerand outer electrodes is limited by abutment within the cavity at eachend of the cell.
 13. An electrochemical cell according to any one ofclaims 1 to 12 wherein the cavity of each cup-shape fitting has astepped internal diameter for defining a first cavity-part leading fromthe open mouth and a second cavity-part of smaller diameter opening fromthe first cavity-part.
 14. An electrochemical cell according to claim 13wherein the ion-permeable tube projects at each end of the cell fromwithin the outer electrode into the first cavity-part of the fitting atthat end to have its annular rim engage with the internal cylindricalwall of the first cavity-part, and the inlet/outlet ports of eachfitting open into/from the first and second cavity-parts respectively.